A model system involving encysting strains of Azotobacter will be used for studying the survival potential of aging populations of non-spore forming bacteria. A series of mutant strains of the Azotobacter and their interacting phages will be used to study the effect of modified internal environment of the cell on the formation of occasional cysts genotypically identical to the rest of the population but of unusual resistance to environmental challenges that destroy the bulk of the population. Our experiments reveal that modifications of the cell envelope are responsible for many of aberrations. We are studying the cell membranes synthesis in mutant and normal cells and the effect of environmental challenge by chemicals that react with these structures. We will extend these studies to enteric and lactic bacteria that in aging and dying populations produce individuals morphologically less dramatic than the Azotobacter cyst but which have undergone some steps in the modifications exhibited by our encysting model.